This invention relates generally to color imaging and more particularly to the use of plural exposure and development steps for such purposes.
One method of printing in different colors is to uniformly charge a charge retentive surface and then optically expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop (REaD) process may be repeated to subsequently develop images of different colors on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). The images may be formed using by a single exposure device, e.g. ROS, where each subsequent color image is formed during a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
In any development system it is very difficult to have 100% purely charged toner, i.e. toner that contains all particles having relatively the same charge level, for developing in the image areas of the photoreceptor. The majority of toner particles in a development system will be at relatively the same charge level, however there is typically the presence of some toner and debris particles having associated charge levels that vary from a moderate to an extreme amount (hereinafter referred to as "wrong-charge" toner) from the charge level of the majority of toner particles, including some toner particles with a charge level polarity which is opposite from the majority. Many factors contribute to the presence of this wrong-charge toner in a development system, including the humidity level, the toner concentration level, and the age, design and overall manufactured quality of the developer material.
The charges associated with the background areas of the photoreceptor are at a much higher or lower level (depending on whether charge area development or discharge area development is being used) relative to the image areas, and therefore the background areas are likely to attract this wrong-charge toner. Thus, a common defect for development systems is the development of this wrong-charge toner in the background non-image areas of the photoreceptor. Since the recharge device sprays charge on both the toned areas and the background areas having wrong-charge toner, transferability of the toner on both of these areas is affected.
In the method of creating multi-color images using the REaD process, the development of subsequent color images onto previously developed images creates a unique set of issues to be addressed. For example, during a recharge step, it is important to level the voltage among previously toned and untoned areas so that subsequent exposure and development steps are effected across a uniformly charged surface. The greater the difference in voltages between a bare photoreceptor area and a toned photoreceptor area following a recharge step, the larger will be the difference in the development potential between the two areas.
In an attempt to achieve optimum conditions for the development of subsequent images onto previously developed images, the operating conditions during recharging often cause wrong-charge background toner and debris to recharge to a level more commensurate with the charge level associated with the toned image. Subsequently, during transfer, the background toner now having a similar charge to the image toner transfers to the support substrate with the toned image, thereby impairing image quality. For example, if the image areas comprise negatively charged toner, and positively charged (wrong-charge) toner is developed onto background areas, during the recharge step, this positive toner may become negatively charged. All toners having a negative charge are then attracted to a support substrate during the transfer step. The transfer of the background toner causes undesirable spots and/or gray areas on the support substrate where clear white background is otherwise desired.
Based on the foregoing, a reliable and consistent manner of recharging the photoreceptor is needed so that the occurrence of transferred background toner is minimized.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 4,791,452 relates to two-color imaging apparatus wherein a first latent image is formed on a uniformly charged imaging surface and developed with toner particles. The charge retentive surface containing a first developed or toned image, and undeveloped or untoned background areas is then recharged by a scorotron charging device prior to optically exposing the surface to form a second latent electrostatic image thereon. An electrical potential sensor detects the surface potential level of the drum to ensure that a prescribed surface potential level is reached.
U.S. Pat. No. 4,819,028 discloses an electrophotographic recording apparatus capable of forming a clear multicolor image including a first visible image of a first color and a second visible image of a second color on a photoconductive drum. The electrophotographic recording apparatus is provided with a conventional charger unit and a second corona charger unit for charging the surface of the photoconductive drum after the first visible image is formed thereon so as to increase the surface potential of the photoconductive drum to prevent the first visible image from being mixed with a second color and also from being scratched off from the surface of the photoconductive drum by a second magnetic brush developing unit.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image.
U.S. Pat. No. 4,033,688 discloses a color copying apparatus which utilizes a light-tens scanning device for creating plural color images. This patent discloses multiple charge/expose/develop steps.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,208,636, discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
U.S. Pat. No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a pre-recharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
Application No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K. K., discloses a color image forming apparatus wherein a first and second charging device are used before exposure and development of an image, where the potential of the image forming unit is higher after passing the first charging device than after passing the second charging device, to eliminate the contrast potential reduction normally caused by applying color toners onto other color toners, and also to prevent toner spray during the exposure process.
The concurrently filed, copending application for patent entitled "Method and Apparatus for Reducing Residual Toner Voltage", Ser. No. (D/92483), by a common assignee as the present application, discloses a voltage sensitive recharge device used for the recharging steps during a color image formation, whose graph of the output current (I) to the charge retentive surface as a function of the voltage to the charge retentive surface (V) has a high (I/V) slope. The high I/V slope recharge device disclosed having an AC voltage supplied thereto, enables an extended time for neutralization to occur at the top of the toner layers.
The concurrently filed, copending application for patent entitled "Split Recharge Method and Apparatus for Color Image Formation ", Ser. No. (D/92485), by a common assignee as the present application, discloses a recharge step between two image creation steps for recharging a charge retentive surface to a predetermined potential pursuant to forming the second of the two images. A first corona generating device recharges the charge retentive surface to a higher absolute potential than a predetermined potential, and then a second corona generating device recharges the charge retentive surface to the predetermined potential. An electrical charge associated with the first image is substantially neutralized after being recharged by the first and second corona generating device.
A number of commercial printers employ the recharge/expose/and develop color image formation process. For example, the Konica 9028, a multi-pass color printer forms a single color image during each pass of the photoreceptor. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC 1 machine employs an AC corona discharge device prior to recharge.